Windows 10 takes up a lot of space during installation. In my machine I have a 120GB SSD for my C drive, installed in a time when SSDs were more than £1 per gigabyte. I've since installed more SSDs, for example, my Steam drive, but never had the need to migrate Windows onto one of the bigger drives. I've distributed all the bulk of the computer system across the other hard drives, like Storage and Media, and never had the need.
After installing Windows 10, however, I fond myself with a mere 25.9GB of C drive space left, which isn't a lot. The reason for this is more than just the new OS.
Windows 10 is a new OS, and some people might not like it. Microsoft have built in a mechanism for those people to go back to Windows 7 or Windows 8.1, to roll back the system to how it was. To do this though, they have to store your entire old system. That takes up space. To see, go to your C drive, look for a folder on the root of the drive called "Windows.old" and see how much space it takes up. For me, it's 4.93GB, a large amount of my 120GB, but not exactly a massive issue for most. "Just delete that" you might think, but there's more to this than initially meets the eye.
Before the installation, I had way more available space. It's not just down to that 4.93GB that I'm seeing the shortfall, so there must be something else, right? So this morning I had a wander onto the new search tool in Windows, but it wasn't yet fully up to speed, it was still indexing and setting itself up. By chance, I typed in "can I delete windows.old. folder" and hit enter without even thinking, and up popped Disk Cleanup. I think this was more that the search wasn't really ready for actual searching yet rather than Windows 10 being smart, but I'll give it the benefit of the doubt here.
Disk Cleanup, the handy little utility, has been around since the dawn of time. People proficient at Windows of old will remember running this to give drives a quick boost and freeing up a bit of space from old installation caches and browser histories. Whilst not always offering a Windows installation a speed boost, as Windows then had to rebuild it's caches anyway, it did free up space.
Running Disk Cleanup revealed where space was really being used. Not only was there the usual Defender files and temporary internet files, but a whole load of other stuff related to the installation of Windows 10.
Running Disk Cleanup alone won't help too much, it will reveal stuff you can delete without administrator permission, and these files don't include the bulk of what you want here. On opening the app, you need to click "OK" to scan C drive, or whichever drive your Windows installation is on, then click "Clean up system files", the icon with the admin shield next to it. It will then re-scan for all files, and you'll find the space hogs.
Previous Windows installs is now 6.4GB, rather than 4.9GB. There's also "Temporary Windows installation files" as well, weighing in at 8.89GB. Total space to gain, just from those two, was 13.8GB. Add that to the update log files, the "RetailDemo Offline Content" and the rest of the crud built up on the machine, we get to a grand total of 15.8GB. Clean up? Please.
So don't go deleting that Windows.Old file straight away. Run Disk Cleanup and get the whole job done at once. If you have already deleted Windows.old though, don't panic. Disk Cleanup will still happily go and tidy up the rest of the mess left behind for you, but you won't see as big a drive gain in one go, and I'm sure Windows 10 will eventually routinely clean away these excess installation files anyway in time, but it doesn't help abate the original panic of finding your C drive 80% full when you first get into Windows 10.